Pyrotechnic flare



Aug. 28, 1962 D. B. FINKELSTEIN PYROTECHNIC FLARE 5 Sheets-Sheet 1 Filed Nov. 24, 1958 Sw e@ wm wm mh., @um

Aug. 28, 1962 D. B. FINKELSTEIN 3,051,085

PYRoTEcHNIc FLARE Filed NOV. 24, 1958 l 3 Sheets-Sheet 2 I mi Aug 28, 1962 D. B. FlNKl-:LSTEIN PYRoTEcHNIc FLARE:

3 Sheets-Sheet 3 Filed Nov. 24, 1958 3,05L085 PYROTECHNlC FLARE Don B. Finkelstein, Redondo Beach, Calif., assignor to Del Mar Engineering Laboratories, Los Angeles, Calif., a corporation of California Filed Nov. 24, 1958, Ser. No. 776,103 4 Claims. (Cl. 102-37.8)

This invention relates to pyrotechnic flares for airborne installations to produce visual energy or infra red ener-gy for observation, tracking and for high energy heat source simulation purposes. While the invention may be adapted for use on various types of airborne vehicles for vari ous specific purposes, it has been initially embodied in a are construction to meet the special requirements for use on a Ktow target. Such an emodirnent of the invention is selected for the purpose of the present disclosure and will provide ladequate guidance for those sldlled in the art who may have occasion to apply the same principles for other specific purposes.

A flare of the type to which the invention is directed consists of `an elongated charge of suitable highly com bustilble material in an elongated metal casing. When the are is in operation, the charge iburns progressively from one end to the other with a traveling combustion zone, and the resulting llame is discharged from the end of the metal casing.

One problem is to cause the charge to burn in the desired progressive manner lof a cigarette without i-gnition occurring in advance of the desired traveling combustion zone. To meet this problem, the charge must be packed snugly in the surrounding metal casing to prevent combustion from creeping along lthe periphery of the charge, and steps must be taken to prevent such excessive heat transfer to and along the Kmetal casing as would cause ignition of peripheral portions of the charge in advance of the desired traveling combustion zone.

The invention meets .this problem by employing a plastic liner for the metal casing and by employing a compact charge, preferably in molded form, which tits snugly into the plastic liner; The snug Ifit keeps combustion from creeping along the periphery of the charge and the material of the surrounding plastic liner is `of sufficiently low thermal conductivity to retard -heat ow to the metal casing and thus keep the temperature of the metal casing far below the adiabatic tempera-ture of .the burning charge. In the burning of a flare exemplifying the invention, the adiabatic temperature of lthe burning charge is on the order of 32003400 K. and the plastic liner is effective to keep the temperature of the metal casing down to a maximum of approximately 1000l300 F. during the combustion cycle.

A further requirement for a Ihare to be used for airborne installations is that the are be of a rugged construction to withstand handling and be of a construction adapted for quick installation. The flare of the present invention is completely enclosed in metal to withstand handling and may lbe installed quickly on an airborne vehicle simply by being seated by one end in a mounting socket or receptacle on the airborne vehicle.

One feature of the invention is the `concept of closing the outer end of the metal casing by suitable closure means and the utilization of an explosive at the outer end of lthe flare for -the dual purpose of blowing out the closure and of igniting the combustion charge. A further feature is the enclosure in the metal casing of the wiring required for electrical ignition of the flare. The wiring is interposed `between the plastic liner and the surrounding casing and is connected Ito exposed contacts at the inner or base end of the casing which complete the tiring circuit by cooperation with complementary contacts in the socket or receptacle in which .the flare is 3,651,685 Patented Aug. 28, 1962 ce Y mounted. Heretofore it has been common to extend a metal tube axially through the `combustion charge to carry the ignition wiring. The new arrangement makes such an axial tube unnecessary and thereby not only simplies the procedure of filling the casing with .the charge lbut also increases the area of the combustion Zone.

When a flare of the present type is to be used on a fragile to-w target, the problem arises of protecting the tow .target against heat damage by the flare. One answer to this problem is the-use of a rugged steel casing for the ilare Ito withstand the temperature of the flare flame and to confine the llame sufficiently to keep the :dame from spreading laterally to damage the tow target in which the flare is mounted. One disadvantage of using a steel casing suiciently rugged for this purpose is the Weight imposed on the tow target. It is contemplated that -four flares will be employed, and four flares of steel construction have a combined weight of ten pounds, tive pounds bein-g dead weight. Weight is an exceedingly impor-tant consideration in the use of tow Itargets, and while a flare load of ten pounds may be carried, there are many instances in which it would be highly desirable to reduce the weight, if only by one pound.

lf a thin aluminum metal casing is substituted 4for Ithe steel casing, the .thin aluminum burns away as fast as the combustion charge and causes damage to the tow target by failure to confine the iiare ame and by the splatter of hot metal against .the tow target body. The present invention has two provisions which make it possible to employ an aluminum casing for the Hare.

One provision is the use of a material Ifor the plastic liner which chars instead of completely disintegrating under .the laction of the heat of the burning char-ge. A charred portion of the plastic liner survives long enough to enclose the traveling combustion zone and is of s-ufiiciently low thermal conductivity to `delay the melting of the aluminum casing. The traveling melting Zone of the aluminum casing lags suiiciently behind the traveling combustion zone to cause the aluminum casing to confine the flare flame with substantial effectiveness.

The Second provision is a thin sheet metal shield surrounding -the aluminum casing 4in radially spaced relation thereto. This outer shield acts as a boundary layer splitter and changes the turbulent boundary layer upstream from the are assembly into a laminar airstream between the shield and the Hare.

It has been found that such a shield may be made of aluminum and may be quite thin if it is to serve merely to create the air cooling action and to shield the tow target body from excessive heat radiation. The aluminum shield must be substantially thicker, however, if it is to perform the additional function of withstanding the hot particles, including hot metal splatter, that are thrown off when the enclosed ilare casing is made of thin aluminum. In this regard, a feature of the invention is the concept of rotating the tow target on its longitudinal axis with the consequent creation of suflcient centrifugal force to cause the hot particles, including hot metal splatter, to be thrown outward away from the tow target body. With the tow target body rotating in this manner, the shield need be only as thick as necessary to carry out the air-cooling function and to protect the tow target body from heat radiation. The shield may be made of aluminum so thin that it disintegrates under the action of the metal splatter, but because of the centrifugal action, the only portion of the shield that disintegrates prematurely is the portion that is positioned radially outward from the tow target body. Such a thin air-cooled shield may reach a temperature of only 250 F. on the side of the shield towards the adjacent tow target body.

The various features and advantages of the invention may be understood byreference to the following detailed description and the accompanying drawings.

In the drawings, which are to be regarded as merely illustrative FIG. l is la View partly in longitudinal section and partly in side elevation showing an embodiment of the invention that employs a steel casing capable of withstanding the heat generated by the flare.

FIG. 2 is a similar view of a second embodiment of the the invention which employs a thin aluminum casing that melts under the heat generated by the burning charge. This ligure further illustrates the manner in which a flare may be mounted in a socket or receptacle on a tow target;

FIG. 3 is a side elevation with portions broken away showing the construction of a flare which incorporates a light sheet metal shield surrounding the flare casing;

FIG. 4 is a perspective view of the base end of the are shown in FIG. 3;

FIG. 5 is an elevation of the same end of the flare;

FIG. 6 is a side elevation of a typical tow target carrying four of the flares of the invention;

FIG. 7 is an elevation of the trailing end of the tow target as viewed along the line 7 7 of FIG. 6; and

FIG. 8 is a fragmentary perspective view of the shielded flare of FIGS. 3, 4 and 5 after the combustion charge is completely consumed, this View showing how the laterally outward portion of the thin shield disintegrates under the Vaction of the hot particles that are thrown centrifugally outward by rotation of the tow target body.

The embodiment of the ilare shown in FIGS. l to 5 is intended to be mounted on a tow target of a well-known construction shown in FIGS. 6 and 7. The tow target in FIG. 6 has a light-weight bulbous streamlined aerodynamic body of revolution, generally designated 10, and is provided with four stabilizing tail tins 11 which 4are angled or bent as indicated at 12 to serve as aerodynamic means to cause the tow target body to rotate on its longitudinal axis in reaction to the air stream. The tow target is connected to a suitable tow line 13 by means of a swivel fitting 14 which permits the tow target to rotate without twisting the tow line. The tow target shown in FIGS. 6 and 7 has four pyrotechnic ares 15 which are mounted in corresponding receptacles 16, the receptacles being faired into the streamlined tow target body.

It is to be understood, of course, that a flare of the present invention may be mounted on an airborne vehicle in any lsuitable manner. In this instance, however, it is contemplated that the flares will tbe mounted on the tow target by bayonet-type connections. For this purpose, each of the receptacles 16 is of cylindrical conguration and,

as shown in FIG. 2,V has an inner transverse partition 1,8

on one side of which is mounted a bayonet socket fitting 20. The partition 18 may comprise a metal disk formed with a circumferential flange 22, which flange is secured to the surrounding wall of the receptacle by suitable rivets 24. The socket tting 2t) has the usual pair of diametrically opposite bayonet slots 25 and at its inner end is provided with a pair of spring-equipped contacts 26. The contacts 26 are connected to a pair of corresponding wires 28 which extend rearward from an electrical fitting 30 on the opposite side of the partition 18. The wires 28 are part of a tiring `circuit on the tow target which may be controlled by suitable means, such as means for remote control by radio signals.

The base ends of the diiterent embodiments of the ares ofv the present invention telescope into the receptacle 16 in the manner shown in FIG. 2 and these base ends are provided with cylindrical bayonet connectors 32 for cooperation with the bayonet socket fittings 20. The bayonet connectors 32 are provided with the usual diametrically opposite lugs 34 for engagement with the bayonet slots 25.

The rst form of the are shown in FIG. 1 has a steel cylindrical casing 35 protected by a liner in the form of an inner plastic shell 36. The plastic liner 36 may be made of any suitable material, such as a polytetralluoroethylene or a phenolic resin either of which chars instead of completely disintegrating under the action of a Vhigh temperature flame. A polytetrafluoroethylene resin for this purpose is available under the trademark Tellon The base end of the steel casing 35 may be closed by a heavy metal disk or base member 38 to which the casing is rigidly united and to which the bayonet connector 32 is in turn firmly united as by welding 40.

It is contemplated that a charge of suitable combustion material will be compacted in snug iit with the plastic liner 36. For this purpose, the combustion charge may comprise a series of molded cylindrical pellets which are dimensioned for close lit with the surrounding plastic shell 36 and which back against a heavy cardboard spacer 46 at the base end vvof the casing. At the outer end of the llare, -a pair of electrical ignition devices in the form of squibs 48 of a well-known type are imbedded in a small body of explosive 50. This body of explosive `is conned between a metal closure disk 52 and 4the combustion .charge comprising the series of pellets 45. The closure disk 52 may be secured in place by crimping of the rim of the steel casing 35, -as indicated at 54. Two electrical squibs 48 'are provided for increasedl reliability and when the body of yexplosive v50 is detonated -by one or both of the squibs, it blows the closure disk 52 free from the 1steel casing 35 and, at the same time, ignites the end face of the -adjacent combustion pellet [45.

As heretofore stated, a feature of .the invention is the manner in which the are is internally wired. In the construction shown, a pair vof contacts 55 carried by the bayonet connector 32 for cooperation with the previously mentioned receptacle contacts 26 are connected to the two squibs 48 in parallel by a pair of wires 56. The wires 56 pass through `a central -aperture 58 in the met-al base disk 38 and extend radially outward between the base disk and the cardboard spacer 46. The two wires 56 extend longi-k tud-inally of the ilare between the plastic liner 36 and the surrounding steel casing 35 and pass through an aperture 60 in the plastic liner for connection to the two squibs 48.v

The manner in which this rst form of the invention serves its purpose may be readily understood from the foregoing description. .It is apparent that with the combustion charge of the flare completely encased in steel, the flare will stand rough handling and may be relied upon to be in good working order when installed on a tow target. The installation of `a flare requires the simple -manipulation of inserting the are into the correspondingy -tow target receptacle 16 and then rot-ating -the ila-re to complete the bayonetengagement between the bayonet connector 32 and the bayonet -socket fitting 20. Rota-v tion of the ilare to complete the bayonet connection places the two electrical contacts 55 of the flare against corre-` sponding spring-equipped contacts -26 of the receptacle, thereby completing the firing circuit for the squibs 48.

When the squibs 48 are energized to detonate the explosive body 50 fat the outer end of the flare for removal of the closure disk 52, the outer end of the elongated explosive charge comprising the series of pellets 45' is ignited and the charge burns progressively with a combustion zone traveling towards the base end of the are. The snug tit of the explosive charge in the surrounding plastic liner 36 prevents migration of combustion in advance of the desired traveling combustion zone and the plastic lliner retards the rate of heat transfer from the combustion zone to the `steel casing 35 to keep the casing from reaching a temperature that would cause ignition of the combustion charge ahead of the advancing combustion Zone. As the elongated combustion charge -is oonsumed, the resulting ilare is confined by the steel casing 35, .the steel casing keeping the are vfrom damaging the tow target body.

Any suitable pyrotechnic material may be employed for the combustion charge. For example, if the purpose of the flare is to produce light in the visible spectrum, the

combustion material may comprise a mixture of magnesium and sodium nitrate. Some other material is required to produce radiation that is primarily in the infra red spectrum. For example, the combustion material may comprise magnesium mixed with a grease of the type sold under the trademark Kel-F.

The second form of the invention shown in FIG. 2 is largely identical to the first form as indicated by the use of corresponding numerals to designate corresponding parts. This construction differs from the iirst described construction solely in the substitution of an aluminum casing 62 for the previously described steel casing 35. The aluminum casing 62 may be of the same Wall thickness as the steel casing 35, for example approximatelyv one-sixteenth of an inch thick. The melting point of aluminum is so much lower than the melting point of steel that the casing 62 melts in response to the heat generated by the burning charge. The aluminum melts readily under the intense heat and in the absence of some retarding expedient, a traveling melting zone `of the casing would coincide With ythe traveling combustion zone of the burning charge.

An important feature of this `embodiment of this invention, however, is that the plastic liner 36 chars and maintains its cylindrical configuration in the presence of the heat of the traveling combustion zone. The charred portion of the liner survives long enough to surround the combustion zone and thus protect the surrounding aluminum casing 62 sufliciently to cause the traveling melting zone of the casing vto lag substantially be'hind the traveling combustion zone. By virtue of this action, a substantial portion of the aluminum casing 62 lined by the charred portion of the liner 36 survives to coniine the flare llame and direct the flare llame axially out of vdirect contact With the adjacent body of the tow target. FIG. 2 shows the charred portion 36a of the plastic liner that surrounds the combustion zone and delays the melting of the aluminum casing.

For some purposes, la flare constructed Ias shown in FIG. 2 may be used Without a surrounding shield. For use on a tow target, however, in relatively close proximity to the tow target body, the shielded construction shown in FIGS. 3, 4 and 5 is desirable.

The are shown in FIGS. 3,'4 and 5 is identical with the flares shown in FIG. 2 except for the addition of a thin aluminum shield `64. The aluminum shield 64 is of cylindrical configuration and surrounds the aluminum casing y62. of the are in spaced relation thereto to form an annular passage 65 foi the ilow of cooling air around the are casing.

The thin aluminum shield `64 may be mounted `on a flare casing in any suitable manner. In the construction shown, the base end of the flare is lanced to form a circumferential series of three straps y66, which straps are offset lfor supporting engagement with the aluminum casing `62. Thus, as shown in FIGS. 4 and 5, each of the straps 66 has a central offset portion that is curved for intimate contact with the periphery of the aluminum casing 62 and is further formed with two radial portions 70 that connect the oset portion with the peripheral portion of 4the shield. As further shown in FIGS. 4 and 5, Ithe aluminum shield -64 may be provided with a series of three screws 72 and corresponding nuts 74 to connect the radial portions 70 at the adjacent ends of the straps `66. It is apparent that the screws 72 may be tightened to constrict the three oifset portions 68 for clamping the shield 64 onto the aluminum casing 62 in radially spaced relation thereto. Preferably the offset portions 68 are additionally bonded to the aluminum casing 62 by suitable adhesive material.

In -the preferred practice of the invention, the second end of the aluminum shield 64 is supported and stabilized by a plastic spacer ring 75 interposed between the inner circumferential surface of the shield and the outer circumferential surface of the aluminum casing 62. The

addition of the spacer ring 75 makes Ithe cylindrical aluminum shield y64 rigid relative to the ilare casing `62 and makes the -aluminum shield capable of withstanding relatively rough handling. The detonation of the explosive body at the outer end of Ithe flare by the previously described electrical squibs and the initial melting of the material of the aluminum casing `62 insures the disintegration or removal of the spacer ring75 to clear lthe annular passage v65 for air ilow therethrough.

'Ihe wall of the aluminum yshield 64 is relatively thin but is adequate in the presence of the cooling air allow to protect the material of the tow target body from excessive radiation of heat from the ilare. The wall of the aluminum shield `64 is not thick enough to withstand the heated particles and splatter metal caused by the burning of the combustion charge and 'the melting of the aluminum casing v62. As heretofore stated, however, the rotation of the tow target creates suflicient centrifugal force to cause the hot particles and molten metal to be thrown laterally outwardly away from 'the tow target body. Consequently, only the radially outward side of the shield 64 is subjected to the deteriorating effects of the burning charge and the molten metal.

It is apparent that the deterioration of this radially outward portion of the aluminum shield 64 is of no consequence since the outwardly directed hot particles and splatter metal do not reach the surface of the tow target body. It is also to be noted that a portion of the shield 64 may melt all the way around the circumference of the shield without the tow target being damaged so long as the melting zone yof the shield lags substantially behind the melting zone of the aluminum casing `62 to shield the tow :target body from direct radiation from the combustion zone. Any splatter metal produced in this way by the melting of the shield `64 Will be thrown outward away from the tow target body by the centrifugal force that results from the spinning of the tow target on its longitudinal axis.

FIG. 8 shows how -at the termination of the burning period, the outer end of the shield `64 may be melted away along the jagged line 76 and further shows how the metal of the shield ion the side that is radially outward from the spinning tow target may be melted away to form a jagged slot `68. Thus the invention utilizes the centrifugal force generated by the rotation of the tow target on its longitudinal axis to make possible a relatively light weight are installation. A set of four ilares constructed in accord with FIGS. 3, 4 and 5 reduces the total weight to seven pounds as compared to ten pounds for -four steelencased flares, and reduces the dead weight 4from tive pounds to one pound. The burning of the ares further reduces the tow target load by Ithe disintegration of the aluminum casings 62 and the disintegration to a lesser extent of the shields 64. This reduction in weight is especially important because it lessens the dit`n`culty of recovering the tow target at the end of the target run.

My description of the selected embodiments of the invention in specific detail will suggest various changes, substitutions and departures from my disclosure within the scope and spirit of the appended claims.

I claim:

1. A flare for use in a tow target to facilitate tracking thereof, comprising: an elongate hollow metal casing; an inner plastic shell lining said casing; a compacted charge of highly combustible material enclose-d by said inner shell; means within said casing at one end thereof for igniting the contiguous end of -said charge to create a traveling combustion zone for progressively igniting said charge longitudinally of the casing with consequent projection of a llame longitudinally from said casing, said charge fitting said inner plastic shell snugly and the plastic material of the inner shell having a low thermal conductivity to retard ignition of the peripheral portions of the charge in advance of the traveling combustion zone, said casing being a thin-walled shell of metal having a relatively low melting poi-nt to be melted? by said'l flame with the melting zonel traveling progressively longitudinally' of the casing, the pl'ast-icmaterial of'said inner shell charring as the shell? is-s'ulbjected to the ame whereby; a charred portion of' theV inner shell survives the heat of the combustion zone to cause the melting zone to lag behind said combustion zone for continuous confinement of t-hecombustion zone by the casing'. v

2. A flare .as set forth in claim 1 which includes a thin- Walled shield to protect the tow target from said flame, said shield being supported with an annular clearance space between the casing and the shield, said shieldv being.

openat both ends-and defining with the casing a lflow pathV for air asv the target istowed to thereby cool the casing during the towedv ilightof the target.

3. A ilare to be mounted on a tow target tofacilitate tracking thereof, which target includes a socket having contact meanstherein electrically connected? int-o anv energizing circuit of the target, comprising: an elongate casing; acharge of highly combustible material carried by said casing; electro-responsive means within saidv casing atone end thereof for ignitingfthe contiguous end of said charge to* create an initial combustion zone for pro-- gressively ignitingsaid charge longitudinally of the casing with consequent projecting of a flame` longitudinally from the said one end of the casing; means carried by the op posite end of the casing for cooperative recept-ive engagement ywith the socket for mounting the flare to the target; contact means externally carried by said last-named endj of thev casing for engagement with the contact means of said socket when said last-named end of the casing ist mounted in the socket;I andconductormeans electrically' connected to the contact means carried by said casing and extending internally of` said casing and electrically connected to said electro-responsiveY means`,`whereby en-4 ergization of the ring circuit of the target electrically ignites the combustible material contiguous of saidelectfro responsive means, thereby Ito create the traveling com bustion, zone and the consequent projectionvof the flame longitudinally: from the casing.

4. Av ilare forl use in an airborne vehicle to facilitate tracking thereof, comprising: an elongated metalcasing; an inner plastic shell lining said casing; a compacted charge yof highly combustible material enclosed by saidk inner shell; means tio ignite" y'one end of said 'charge at one end of said casing for burning of the charge to create al combustion Zone with? the combustion zone traveling progressively longitudinally o'f `the charge `with consequent projection of a ar'ri'e longitudinally from said 'casing, said charge 'tting said inner plastic shell snugly and the plastic material of the inner shell being of low thermal conductivily to prevent ignition of the peripheral portions of the charge in advance of the traveling combustion zone, said casing being a thin-walled shell of metal having' a relatively low melting point to be melted by said flame with the melting zone traveling progressively longitudinally of the casing, said inner shell being made of a plastic material that chars in contact with a hightemperatureame whereby a charred. portion of the inner shell survives the heat of the combustion zone to' cause said melting.` zone to lag behind said' combustion zone for con-Y tinuous confinement of the combustion Zone by the casing;` a thin-walled shield to protect the airborne vehicle from said flame, Vsaid'shi'el'dA bein'g' supported with an annular? clearance space between t-he casing andl the shield, said? shield being open at both ends to ow therethrough of' the slipst'reanfr of the aerial vehicle to be cooled thereby, said shield being a sheet' metal cylinder, portions of said cylinderV being lanced and offset radially inward" for sup: portingengagement with the periphery of said casing, paired opposed portions of said offset portionsbeing in-y terconnected by'screw means. to tighten they grip' of the shield on` the casing.

References Cited inzthele of this patent Uhl-NED STATES PATENTS 2,006,271 Hitt ]une 25, A1935 2,307,707 Nichols` Y Jan. 5, 1943A 2,448,521 Dwyer Sept. 7, 1948 2,539,404 Crutchlield et al Jan. 30, 1951V 2,543,079 Veek Feb. 27,. -1: 2,631,796 Williamson et a1 Mar. 117, 1953 2,670,601 Williams et a1 Mar. 2,1954a 2,791,178 Thompson May 7, 15957y 2,813,719 Hopper l Nov. 1'9, 1957 2,821,396 Seeley `Ian. 28, 1958 2,829,596 Loedding Apr. 8, 1958 

1. A FLARE FOR USE IN A TOW TARGET TO FACILITATE TRACKING THEREOF, COMPRISING: AN ELONGATE HOLLOW METAL CASING; AN INNER PLASTIC SHELL LINING SAID CASING; A COMPACTED CHARGE OF HIGHLY COMBUSTIBLE MATERIAL ENCLOSED BY SAID INNER SHELL; MEANS WITHIN SAID CASING AT ONE END THEREOF FOR IGNITING THE CONTIGUOUS END OF SAID CHARGE TO CREATE A TRAVELING COMBUSTION ZONE FOR PROGRESSIVELY IGNITING SAID CHARGE LONGITUDINALLY OF THE CASING WITH CONSEQUENT PROJECTION OF A FLAME LONGITUDINALLY FROM SAID CASING, SAID CHARGE FITTING SAID INNER PLASTIC SHELL SNUGLY AND THE PLASTIC MATERIAL OF THE INNER SHELL HAVING A LOW THERMAL CONDUCTIVITY TO RETARD IGNITION OF TH PERIPHERAL PORTIONS OF THE CHARGE IN ADVANCE OF THE TRAVELING COMBUSTION ZONE, SAID CASING BEING A THIN-WALLED SHELL OF METAL HAVING A RELATIVELY LOW MELTING POINT TO BE MELTED BY SAID FLAME WITH THE MELTING ZONE TRAVELING PROGRESSIVELY LONGITUDINALLY OF THE CASING, THE PLASTIC MATERIAL OF SAID INNER SHELL CHARRING AS THE SHELL IN SUBJECTED TO THE FLAME WHEREBY A CHARRED PORTION OF THE INNER SHELL SURVIVES THE HEAT OF THE COMBUSTION ZONE TO CAUSE THE MELTING ZONE TO LAG BEHIND SAID COMBUSTION ZONE FOR CONTINUOUS CONFINEMENT OF THE COMBUSTION ZONE BY THE CASING. 